Mercury (Hg) is a toxic metal that can disrupt natural enzymatic processes and cause cellular stress1a. In addition to being toxic as inorganic HgII, humans and wildlife are also exposed to the potent neurotoxin monomethylmercury (MMHg). Upon deposition from the atmosphere, HgII first interacts with inorganic and organic ligands under oxic conditions. Furthermore, whereas microbial HgII methylation to toxic MMHg occurs under anoxic conditions, recent reports suggest that methylation may not solely be limited to anoxic microbial habitats1b. Therefore, a comprehensive approach to studying HgII pathways from the atmosphere to methylation sites requires that HgII speciation and bioavailability to microbial cells be studied over a wide range of environments, including oxic freshwaters213. Such studies, characterizing dissolved organic matter (DOM) on a molecular level and how it interacts with HgII are not known to have previously been disclosed. DOM is a complex mixture of heterogeneous material deriving from a variety of autochthonous and allochthonous sources that can, for example, act as a microbial nutrient source, affect light penetration in solution and chelate metals3-7.
DOM has a very diverse and dynamic composition that, while not wishing to be limited by theory, may interact with HgII in several ways. First, the binding of HgII to DOM can decrease its mobility and bioavailability via the formation of metastable structures with large hydrophobic DOM molecules5-7. Alternatively, the binding of Hg to small molecules such as amino acids or peptides (e.g., cysteine or glutathione (GSH)) has been shown to increase Hg bioavailability8. DOM hydrogenation and oxygenation has been previously correlated to microbial uptake of DOM nutrient sources9. For example, DOM high in hydrogenated (larger H/C, lower O/C) nitrogen containing compounds correspond to more bioavailable amino acid rich material whereas highly oxygenated (larger O/C, lower H/C) carbon sources are less favorable microbial nutrient sources9-19. Sulfides (free or associated with DOM) are predicted to be an important ligand for HgII under anoxic conditions that also favor MMHg formation, but strong interactions between HgII and DOM in oxic waters can affect subsequent binding to sulfides7. This is important in stratified aquatic ecosystems where an oxic water column overlies an anoxic zone11-14. The binding of Hg to DOM is controlled by a group of homologous structures within DOM containing reduced sulfur species (R-SH), but understanding how composition and size of organic ligands influence Hg bioavailability has also been lacking15-19.